1. Field of the Invention
The present invention relates to a method of and apparatus for detecting whether a wide range air-fuel ratio sensor has been activated or not.
2. Description of the Related Art
For controlling an air-fuel ratio mixture to be supplied to an engine in a way as to allow the air-fuel ratio to be maintained at a target value (i.e., stoichiometric) and thereby reducing the concentration of CO, NOx and HC in the engine exhaust gases, it is known to carry out a feedback control of a quantity of fuel to be supplied to the engine. For such feed-back control, a .lambda. (lambda) sensor whose output changes abruptly or sharply (i.e., stepwise) in response to a particular oxygen concentration, i.e., a theoretical air-fuel ratio mixture and a wide range oxygen sensor or air-fuel ratio sensor, whose output changes smoothly and continuously (i.e., not stepwise) in response to a variation of the air-fuel ratio from a lean mixture mode or range to a rich mixture mode or range are mainly used. The wide range air-fuel ratio sensor is, as mentioned above, capable of detecting the oxygen concentration in an engine exhaust gas continuously and improving the feedback control accuracy and speed, and is thus used in case the higher-speed and more accurate feedback control is required.
The wide range air-fuel sensor is provided with two cells made of oxygen ion conductive solid electrolytic bodies, which are disposed so as to oppose to each other with a certain interval or gap (measurement chamber) therebetween. One of the cells is used as a pump cell for pumping out the oxygen from or into the gap between the cells. The other of the cells is used as an electromotive force cell for generating a voltage depending upon a difference in the oxygen concentration between an oxygen reference chamber and the above gap. The pump cell is operated in such a manner that the output of the electromotive force cell is constant, and the current supplied to the pump cell to this end is measured for use as a value proportional to a measured oxygen concentration. An example of such a wide range air-fuel ratio sensor is disclosed in U.S. Pat. Nos. 5,174,885 and 5,194,135.
The above described feedback control for reducing the noxious components contained in the exhaust gases starts after warm-up of the engine is completed. This is because the wide range air-fuel ratio sensor is not active or operable until it is heated up to a predetermined temperature to make higher the activity of its oxygen ion conductive solid electrolyte. For this reason, a heater is provided to the wide range air-fuel ratio sensor in order to make it operable as soon as possible after starting of the engine.
In this connection, before the feedback control by means of the above described wide range air-fuel ratio sensor starts, the air fuel ratio is regulated to a rich mode so that the exhaust gases with a relatively high concentration of CO and HC are emitted. In order to finish the emission of such exhaust gases with a high concentration of the noxious components within a short time, judgment on whether the wide range air-fuel ratio sensor has been activated or not is made by applying a predetermined current or voltage to the electromotive force cell for measurement of the resistance in order that the wide range air-fuel ratio sensor can be put into action as early as possible after starting of engine.
Referring to FIG. 8A, a prior art technology on such judgment will be described. FIG. 8A is a graph of a voltage across an electromotive force cell, resulting when a constant current is passed through the electromotive force cell, as a function of a time elapsing after heating of the electromotive force cell starts. The electromotive force cell decreases in resistance gradually as it is heated for a longer time. That is, the temperature of the electromotive force cell is assumed on the basis of the resistance, and judgment on whether the electromotive force cell has reached a predetermined temperature at which the electromotive force cell becomes active or operable is made on the basis of the assumed temperature. In this instance, when the voltage of the electromotive force cell has reached a value Vs1, it is judged that the electromotive force cell (i.e., wide range air-fuel ratio sensor) has reached a temperature at which it becomes active, and after a margin of about 10 seconds elapses the operation of the wide range air-fuel ratio sensor is made to start.
In this connection, the reason why the wide range air-fuel ratio sensor is made to operate after a margin of about 10 seconds elapses in the above described prior art technology is that the voltage Vs of the electromotive force cell varies depending upon a variation of the surrounding atmosphere. That is, an electric potential is generated in the electromotive force cell on the basis of an oxygen concentration in the surrounding atmosphere, so the temperature that the electromotive force cell has at the time of its voltage having reached the above described value Vs1 varies depending upon whether the surrounding atmosphere shows a rich mixture or a lean mixture. For this reason, assuming that the electromotive force cell does not yet become actually active or operable even when its voltage has reached the above described Vs1, heating is continued for further 10 seconds or so.
A further accurate technology of detecting whether the electromotive force cell has become active or operable is disclosed in Japanese patent provisional publication No. 4-313056. By this technology, as shown in FIG. 8B, application of a current to the electromotive force cell is periodically stopped or suspended, whereby the resistance of the electromotive force cell is measured on the basis of the voltage fall Vsd3 resulting when the application of current to the electromotive force cell is stopped or suspended, and judgment on whether the electromotive force cell has become active or operable is made based on the resistance measured as above. That is, the voltage Vs of the electromotive force cell is the sumn of the result obtained by multiplying the resistance of the electromotive force cell and the current passed through the same, and the internal electromotive force (hereinafter also referred to simply as electromotive force) of the electromotive force cell, so the voltage fall at the time of stoppage or suspension of the supply of current to the electromotive force cell is dependent only on the resistance value of the electromotive force cell. Thus, the temperature of the electromotive force cell can be measured irrespective of the internal electromotive force which varies depending upon a variation of the surrounding atmosphere.
However, it was found that the above described prior art technology was insufficient in accuracy in detecting the resistance of the electromotive force cell. That is, the voltage across the electrodes at the opposite side surfaces of the electromotive force cell after the current or voltage to be applied to the electromotive force cell is suspended or stopped, is not necessarily constant but varies momentarily even during a quite short time (e.g., 1 ms or less), so the prior art of FIG. 8A-8B encounters such a problem that it is unclear that at what time after application of the current or voltage is suspended or stopped, measurement of the voltage across the electrodes at the opposite side surfaces of the electromotive force cell should be made for accurate detection of the activity of the sensor.
The cause of voltage variation within a quite short time has not yet been clearly explained but may roughly be assumed as follows.
As shown in FIG. 1, an electromotive force cell 24 has such a structure as having electrodes 22 and 28 at opposite side surfaces of a solid electrolytic body and is not a simple, pure resistance in view of an equivalent circuit but has characteristics of complex impedance resulting from a combination of a resistance and a latent electrostatic capacity. Accordingly, as described above, a response to a sudden, step-like variation of applied voltage (stoppage of application of voltage) does not occur in a sudden, step-like manner but exhibits a complex variation resulting from a combination of several exponential variations.
In this instance, if judgment on the activity is made by using a voltage detected at a suitable timing, there may possibly be caused a problem that the output of the wide range air-fuel ratio sensor is inaccurate or judgment on the activity is delayed with the result that the wide range air-fuel ratio cannot function properly or desirably.